In injection molding, the process of forming the molded article usually is divided into three phases, filling, injection pressing and holding pressing. During injection pressing, the cavity volume is filled and the thermoplastic material is consolidated, while a great pressure increase takes place in the molding tool. This pressure reaches its highest value at the end of the injection pressing phase.
In the subsequent phase, the molded article cools off. At the same time, its volume decreases. During this phase, further material is supplied at a constant pressure level to the material that is already in the cavity.
If the process is to be conducted in an optimum manner, the selection of the change-over point, adhering precisely to this change-over point and the reproducibility in each injection molding cycle are of great importance. Fluctuations in the change-over point have an effect on the internal pressure of the molding tool and, with that, on the quality of the molded part.
If the change-over point occurs too early or at too low a switching pressure, there is a break in the internal pressure of the molding tool, which results in surface markings due to a brief, partial freezing of the flow front.
If the change-over comes too late, a pressure peak results with the negative consequence of a reduction in the quality of the molded part in the form of flash. Excessive pressure can also lead to damage to the molding tool.
German published patent application No. 2,443,930 discloses a method with appropriate apparatus, in which the change-over point is shifted by shifting the mold gap splitting time or the mold gap splitting path and the total energy supplied to the mold cavity is used as a basis for the correcting variable that determines the degree, to which the mold is filled in the holding phase. To carry out the method, it is proposed that a measuring element for the mold gap or the internal pressure of the mold be disposed in the region of the cavity.
The proposed method and its equipment are not only sluggish, but are also disposed at places, at which the molding tool may be damaged and which, moreover have a negative effect on the quality of the molded part.
In the specification of the European patent publication No. 233 548 A 2, a method is disclosed for regulating and controlling and/or monitoring the injection molding process and equipment is shown for carrying out the method, in which the pressure and time signals are used to control, regulate and/or monitor the molding process. The identified deviations are used to change the process parameters, such as the injection speed, pressure or stroke, the mass temperature and/or the molding tool temperature for the next cycle of the machine.
This method is relatively sluggish and has, moreover, the disadvantage that changes are made in the process parameters, which are not involved causally in the shift of the change-over point from injection pressure to holding pressure and, moreover, are shifted from their optimum operating point to less favorable areas.
It is to be understood that reference in the specification and the claims to "pressure", also means reference alternatively to the prevailing force which is interchangeable proportional to the pressure, and reference to "time", also means reference alternatively to the path which is interchangeably proportional to the time.